If countries scrap nuclear plants, which emit no greenhouse gases blamed for global warming, they may turn to the fossil fuels that experts call the main culprit behind climate change. Environmental activists counter that the tragedy may prove a defining moment, a window of opportunity to strike a decisive blow against both.

“It’s a false choice to give the public an alternative between a climate change disaster or a nuclear disaster. We need renewable energy,” said Tove Maria Ryding of the environmental group Greenpeace. “Now, we can either have a kick back or a leap forward.”

Christiana Figueres, the U.N.’s top climate change official, said that all countries are reviewing nuclear policies in the wake of Japan’s crisis.

“It remains to be seen what they decide,” she said at a 173-nation conference running through Friday in Bangkok. The gathering aims to build on a climate summit held last December in Cancun, Mexico.

Figueres and others are concerned that pledges made by governments to reduce greenhouse gas emissions so far equal only 60 percent of what scientists say is required by 2020 to keep temperatures from rising more than 2 degrees Celsius (3.8 F) above preindustrial levels.

A swing back to fossil fuels presumably would worsen the effects of climate change, which many scientists say causes a melting of polar ice caps and glaciers, a rise in sea levels and extreme weather.

Before a tsunami ravaged Japan’s Fukushima Dai-ichi nuclear complex last month, the Paris-based International Energy Agency had estimated that nuclear plants would add 360 gigawatts of generating capacity to the global inventory by 2035.

After the accident, that projection has been cut in half, agency chief economist Fatih Birol said, citing the pressure to halt new nuclear plants and phase out older ones sooner than planned.

The gap is likely to be filled equally by renewable energy, coal and gas. The result will mean an additional 5 percent — or 500 million tons — of carbon dioxide emitted globally by 2035, Birol said in an interview.

“The doors are fast closing on the 2-degree target, and with a decrease in nuclear energy it makes it even more difficult,” Birol said. “It’s all bad news — cost of energy will increase, energy security and diversification decrease and carbon emission will go up.”

Experts wonder whether countries really will slash nuclear power as much as their initial reactions to the Fukushima tragedy suggest, and if so, whether they will they speed toward renewables or simply burn more coal.

Ryding said she is concerned that several governments, already backtracking on earlier pledges to reduce emissions, may use Fukushima as an argument to do even less.

Birol of the IEA, which advises governments on energy policy, says some world leaders may have been “too abrupt” in moving away from nuclear energy in wake of the Japanese disaster.

“When we have all the input from Fukushima, I am sure that policy makers will take another look, especially given the big economic stakes,” he says.

The scene is hardly uniform around the globe, where there are currently 507 nuclear power plants in operation or under construction and where oil, coal and gas still provide the bulk of energy in most countries.

In Japan, climate negotiators expect a greater, short-term reliance on fossil fuels to fill the nuclear power gap and are concerned the country could reduce its pledge to cut emissions by 2020 — from 25 percent down to 20 percent.

But Prime Minister Naoto Kan said alternative new energy would become “a major pillar” after the Fukushima accident.

“Taking this as a lesson, we will lead the world in clean energy such as solar and biomass, as we take a step toward resurrection,” he told lawmakers last week.

China, the world’s no. 1 emitter of greenhouse gases, has ambitious plans to move away from coal plants that provide 70 percent of its energy and go toward clean alternatives. It may also scale back its nuclear program in light of the Japan emergency, Chinese climate envoy Xie Zhenhua said.

“I believe this accident will have some impact on the development of nuclear power not only in China, but also the rest of the world,” he told reporters in Australia last week.

U.S. President Barack Obama has defended nuclear energy, but also strongly supports development of solar cells, clean coal and biofuel technology.

The most dramatic developments are likely to occur in Western Europe. Germany had planned to phase out nuclear power over 25 years. But the Fukushima crisis — which Chancellor Angela Merkel called a “catastrophe of apocalyptic dimensions” — has accelerated those plans.

The government almost immediately took seven of its 17 reactors offline for three months of safety checks. Most of Germany’s leaders now seem determined to swiftly abolish nuclear power, possibly by 2020, and are willing to pay for intensive development of renewable energy, already a major industry in Germany.

The country currently gets 23 percent of its energy from nuclear power — about as much as the U.S. Germany’s Environment Ministry says that in 10 years, renewable energy will account for 40 percent.

That kind of plan would not work for countries such as France, which relies on nuclear for 70 percent of its power and has no intention of shifting, but could provide a map for other countries, activists say.

Sven Teske, Greenpeace’s renewable energy director, said Germany was able to fill its energy gap left by idled nuclear plants with wind and solar power, though it has had to import some energy from nuclear-reliant neighbors.

“Switching to renewable is a matter of years, not decades,” Teske said.

The International Panel on Climate Change, a scientific body set up by the UN and winner of a Nobel Peace Prize, says a global phase-out of nuclear power plants is feasible at moderate costs and without taking away from climate change efforts.

Artur Runge-Metzger, a European Union climate change official in Bangkok, said the issue is often seen in terms of “two kinds of evils.”

“On the one hand you say we can’t use nuclear energy because we might have nuclear disasters, but everybody at the table is also saying if we have climate change it is also going to lead to disaster,” he said. “So we have to find a way forward.”

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Associated Press reporters Mari Yamaguchi in Tokyo and Juergen Baetz in Berlin contributed to this report.